Combination current sensor and relay

ABSTRACT

A protection device for monitoring current in a power cable to an electrical device and for controlling a remotely located starter for the electrical device in response to a system controller. The device includes a transformer magnetically linked with the power cable to produce a voltage signal in response to the presence of a changing current within the power cable. An input circuit located in a single housing together with the transformer is electrically connected to the transformer so as to receive the voltage signal. The input circuit produces, in response to the voltage signal, either a first signal or a first circuit condition at the output terminal of the input circuit, representative of the changing current in the power cable. A switch circuit also in the same container with the transformer has a terminal for sensing either a second signal or a second circuit condition of the remotely located system controller. The switch circuit has a second terminal to provide either a third signal or a third circuit condition effective to control the starter, in response to sensing one of the second signal and the second circuit condition. All of the transformer, the input circuit, and the switch circuit are located in a single unitary package which is easily mounted and adjusted to a desired position.

BACKGROUND OF THE INVENTION

The present invention relates to a single protection device, including atransformer, an input circuit, and a switch circuit, suitable foraddition to an electrical power system that includes a control panelelectrically interconnected with a starter to control an electricaldevice. It is desirable to monitor the performance of devices such aselectric motors and to control them through remotely located controlpanels. While this has long been possible, it has been costly to installthe required sensors and controls.

Springer, U.S. Pat. No. 4,885,655 discloses an integratedcapacitor-start, induction-run electric motor starter and protectioncircuit specifically suitable for a water pump. The circuit requiresphysical electrical connection to the power cables to sense the phaseangle between the voltage and the current applied to the motor toindicate when it is operating without a load. When such a conditionoccurs, the circuit triggers an activator coil to de-energize the motorby opening a switch member. The integrated starter and protectioncircuit is suitable only for induction machines.

Libert, U.S. Pat. No. 4,887,018, discloses a start-up circuit forgradually starting a multiphase motor and which also includes detectorcircuits for detecting various fault conditions and for disconnectingpower from the motor in response to fault conditions. Integrating themotor starter and protection circuit into one device makes it unsuitablefor addition to existing power systems to detect fault conditions withinpower cables.

Fluckiger, U.S. Pat. No. 5,359,273, discloses a load control circuit forcontrolling the load of an asynchronous motor. The circuit includes apair of capacitors, one of which can be connected in parallel with theother by a switch. Control over whether one or two capacitors issupplied as a load to the motor is provided by comparing signals from acurrent sensor and a voltage sensor in a comparator. Depending upon therelative size of the compared signals, the switch is turned off or on.

Domshy, et al., U.S. Pat. No. 3,593,078, discloses a starting andoperating control circuit that includes voltage sensors and currentsensors to start a motor and limit the power supplied to it.

One design of a starter includes an integrated starter and protectioncircuit to detect fault conditions of an associated electrical deviceand, in response to detecting a fault condition, disable the electricaldevice. When a starter with an integrated starter and protection circuitis used to control devices in an environment in which the electricaldevices are not dependent upon other machines, or their operation doesnot impact other devices, the inclusion of the integrated starter andprotection circuit is an effective way to protect the associatedelectrical device. However, when a starter including such an integratedprotection circuit is used in a system where the associated electricaldevice is interdependent with other electrical devices, use of such astarter to independently enable and disable the electrical device mayresult in devastating effects to the entire system. In other words,where several electrical devices combine to produce a result, as insystems that includes a computerized control panel or system controller,it is desirable for the control panel or system controller to maintaincontrol over the status of the electrical devices to minimize potentialproblems.

However, many starter designs do not include an integrated protectioncircuit and also include no-fault detection. To provide fault detectionfor electrical devices, a current sensor surrounding the power cable tothe electrical device may be used to sense the electrical load current.The current sensor may be electrically interconnected between a controlpanel and the power cable to provide a signal to the control panelrepresentative of the current within the power cable. Suitable currentsensors are known, such as the current sensor sold by Veris Industries,Inc., under the trademark HAWKEYE 700. It is desirable to locate such acurrent sensor within a housing where a starter for the electricaldevice is located.

A relay is typically electrically interconnected between a remotelylocated control panel and a starter to receive a control signal from thecontrol panel and in response to provide an indication to the starter.Like the current sensor, such a relay may be located within a starterhousing.

The relay and current sensor in the past have been separate individualdevices, each requiring a mounting location that may not be availablewithin the starter housing due to limited space. Additionally, it takesadditional time to install two separate devices and requires stockingtwo separate replacement parts for use in the event of device failure.Also, troubleshooting likely requires the inspection of both devices.

What is desired, therefore, is a protection device that is suitable foraddition to existing electrical systems that operate in an environmentincluding a control panel and a starter controlling the operation of anassociated electrical device. The protection device should be compact,inexpensive, and fast to install.

SUMMARY OF THE PRESENT INVENTION

The present invention overcomes the aforementioned drawbacks andshortcomings of the prior art by providing a protection device formonitoring current in a power cable to an electrical device and forcontrolling a starter for the electrical device in response to aremotely located system controller. The protection device includes atransformer magnetically linked with the power cable connected to theelectrical device. The transformer produces a voltage signal in responseto the presence of a changing current within the power cable. An inputcircuit located proximate to the transformer has an output terminal andis electrically connected to the transformer so as to receive thevoltage signal. The input circuit produces, in response to receiving thevoltage signal, one of a first signal representative of the changingcurrent and a first circuit condition at the output terminal of theinput circuit representative of the changing current in the power cable.A switch circuit has a first terminal for sensing one of a second signaland a second circuit condition of the remotely located systemcontroller. The switch circuit has a second terminal to provide one of athird signal or a third circuit condition, in response to sensing one ofthe second signal and the second circuit condition. The third signal andthe third circuit condition is effective to control the starter when thestarter is electrically connected thereto. All of the transformer, theinput circuit, and the switch circuit are located in a single unitarypackage.

By locating the sensor circuit, which is a combination of thetransformer and the input circuit, and the switch circuit proximate toone another within the single package, significant advantages arerealized. Due to the limited available space within a starter housing,the single package is more desirable. The expense of manufacturing asingle package is frequently less than manufacturing two separatepackages. The time required to install both the switch circuit and thesensor circuit is reduced by only requiring installation of one combinedpackage. A reduction in the number of backup parts and troubleshootingtime is also realized.

In a preferred package a mounting bracket includes a slide arrangementto support the transformer so that its position can be adjusted easilyto the best location with respect to the power cable.

The foregoing and other objectives, features, and advantages of theinvention will be more readily understood upon consideration of thefollowing detailed description of the invention, taken in conjunctionwith the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation of an electrical system thatincludes a protection device according to the present invention, acontrol panel, and a starter controlling the operation of an associatedelectrical device.

FIG. 2 is a pictorial representation of the protection device of FIG. 1,showing a transformer, an input circuit, and a switch circuit.

FIG. 3 is an electrical schematic diagram of a transformer and inputcircuit which is a portion of a first embodiment of the presentinvention, suitable to provide a full scale 0 volt to 5 volt outputsignal.

FIG. 4 is an electrical schematic diagram, of a transformer and inputcircuit which is a portion of a second embodiment of the presentinvention suitable to provide an open circuit or short circuitcondition.

FIG. 5 is an electrical schematic diagram of a transformer and inputcircuit which is a portion of third embodiment of the present invention,suitable to provide a 4-20 ma output signal.

FIG. 6 is an electrical schematic diagram of a switch circuit includinga relay, which is a portion of one embodiment of the present invention.

FIG. 7 is an electrical schematic diagram of a switch circuit includinga triac, which is a portion of another embodiment of the presentinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to FIG. 1, an electrical system 10 includes a control panel 20that utilizes a digital computer to provide effective control of manyassociated electrical devices. The centralized control panel 20determines the effects on the entire system 10, or a portion of thesystem 10, of enabling or disabling an electrical device. For example,such associated electrical devices may include motors, pumps, fans,valves, generators, switches, lights, etc. One type of control panel 20is generally known as a programmable logic controller, such as thosesold by Allen Bradley.

A starter 22, designed to start (energize) and stop (de-energize)remotely located electrical devices, is electrically connected to anassociated electrical device 24 by three power cables 23a, 23b, and 23c.Each starter 22 is usually located within an individual starter housing26 which is a part of a substation. Most substations are not large, soit is desirable to reduce the size of the housing 26, so as to maximizethe number of housings 26 that may be located within the substation.Accordingly, the housing 26 is normally designed to be only slightlylarger than the enclosed starter 22, and so there is only limited spacewithin the housing 26 in which to place additional devices, such asprotection devices.

Referring to FIG. 2, a protection device 35 includes both a currentsensor, including a transformer 34 and an input circuit 50, and a switchcircuit 60 within the single package 30. The package 30 is preferablyslidably mounted on a support 31 mounted within the housing 26. Byplacing the transformer 34, input circuit 50, and switch circuit 60proximate to one another, within the single package 30, it isconsiderably easier to locate the device 35 within the limited space ofthe starter housing 26. Additionally, installing only the single package30 requires less installation time than installation of separate devicesto perform each of the desired functions, and the expense ofmanufacturing, packaging and shipping a single device is less than fortwo separate devices. A reduction in the number of backup parts andtroubleshooting time is also realized.

The package 30 defines a central opening 32 through which the powercable 23c is routed. Surrounding the central opening 32 is a toroidalsensing transformer 34 to sense the changing current within the powercable 23c. The toroidal sensing transformer 34 is preferably awire-wrapped magnetically permeable toroidal core, normally made ofiron, encircling the respective power cable. Thus, the wire wound on thetoroidal sensing transformer 34 is the secondary winding, while thepower cable 23c, or a parallel shunt current divider (not shown), is theprimary winding of the toroidal sensing transformer 34. Changing currentin the power cable 23c induces a changing electromagnetic field aroundthe power cable 23c, which in turn induces a magnetic flux in themagnetically permeable core. The magnetic flux in the core induces inthe wire windings on the toroidal core a voltage representative of thecurrent in the power cable. An exemplary sensing transformer has thefollowing construction: core material made by Arnold Engineering, ofNorfolk, Neb., of 0.012 silectron, 3% silicon steel, grain oriented,with an outside diameter of 1.375 inches, an inside diameter of 1.125inches, strip width of 0.500 inches, strip thickness of 0.012 inches, anepoxy powder coating of 0.010 to 0.030 inches thick, a nylon overcoatwound on the metal core, and a #35 AWG size wire coated with a heavypolyurethane wound 1,800 turns as a secondary winding.

Such a sensing transformer with a core of magnetically permeablematerial, such as iron, generates a voltage signal reasonably accuratelyrepresentative of the current in the power cable over a certain normalload range. However, iron and other magnetically permeable materialshave hysteresis and other nonlinear responses to changing magneticfields that result in a nonlinear relationship between current in thepower cable and the voltage signal produced in a transformer coil havingsuch a core. The nonlinearity of such responses is especiallysignificant with large variations in load current and frequency. Toprovide a more linear measurement of power, "air core" transformers havebeen designed using wire wrapped on a core made of material having a lowmagnetic permeability, such as one of plastic or nylon. Without amagnetically permeable core, however, the transformer winding generatesrelatively small voltage levels in response to power cable currents. Anexemplary air core transformer has the following construction: core ofnylon, outside diameter of 1.375 inches, inside diameter of 1.125inches, strip width of 0.500 inches, and a #35 AWG size wire coated witha heavy polyurethane, wound 4,000 turns as a secondary winding. Examplesof circuitry suitable for use with an "air core" transformer aredisclosed in U.S. patent application Ser. No. 08/300,732, assigned tothe same assignee, and incorporated herein by reference.

The ends of the secondary winding 40a and 40b of the transformer 34 areelectrically connected to an input circuit 50. The input circuit 50 isdesigned to convert the voltage signal received from the transformer 34to either a signal representative of the changing current in the powercable or a circuit condition at the output terminal 41a and 41brepresentative of the changing current in the power cable. The signal orcircuit condition is provided to transmission lines 54 and 56 which areconnected to the control panel 20. For example, the signal could be acurrent signal, voltage signal, or some sort of frequency modulation,amplitude modulation, or digital encoding. The circuit condition, forexample, could be a short circuit, open circuit, or other suitable typeof condition. The input circuit 50 can be designed and constructed inany manner, so long as it converts the voltage signal output from thetransformer 34 to an appropriate corresponding signal or circuitcondition. Several exemplary input circuit designs are described below.A light emitting diode 58 is electrically connected to the input circuit50 and is illuminated when current is sensed within the power cable. Apotentiometer 59 allows adjustment of a threshold level within the inputcircuit 50 of the sensed voltage from the transformer 34.

The use of the control panel 20 or system controller provides automatedcontrol over the electrical system 10. The control panel 20 receives thesignal from the input circuit 50 or determines the circuit condition ofthe input circuit 50 via a pair of transmission lines 54 and 56. Thecontrol panel 20 in response to receiving the signal or determining thecircuit condition of the input circuit 50 analyzes the signal or circuitcondition to determine information such as power consumption,overcurrent, overvoltage, undercurrent, undervoltage, frequency, spikes,harmonics, etc. From this information the control panel 20, among otherthings, may determine that the electrical device 24 should be disabledor enabled. For example, if the current sensor indicates that a motor(not shown) for a pump is malfunctioning, then the control panel 20 mayhave that motor deactivated. If deactivation of that motor would alsoimpact another device, such as an auger within a storage tank supplyingfluids to the pump, then the control panel 20 may also deactivate themotor for the auger.

The control panel 20 is electrically connected to a switch circuit 60 bya pair of transmission lines 61 and 63. The switch circuit 60-is locatedproximate to the transformer 34 and input circuit 50. The switch circuit60, transformer 34, and input circuit 50 are all enclosed within thesingle package 30. The package is preferably mounted within the starterhousing 26. The switch circuit 60 includes any suitable switchingdevice, for example, a triac or a relay, as will be described below. Thetriac or relay is powered by a 24 volt AC or DC signal through thetransmission lines 61 and 63. The power on the transmission lines 61 and63 closes the circuit through the switch circuit 60 and maintains ashort circuit between the output terminals 67a and 67b of the switchcircuit 60. When power ceases to be supplied to the switch circuit 60,the output terminals 67a and 67b of the switch circuit 60 areelectrically isolated from each other (open circuit). With the outputterminals of the switch circuit 60 in an open circuit condition when thetransmission lines 61 and 63 are not powered, a safety feature for thestarter 22 is provided in the event of power failure to the controlpanel as will be described below. Alternatively, the switch circuit 60could be designed to be controlled by any type of suitable signal orcircuit condition.

A pair of wires 70 and 72 are connected between the output terminals 67aand 67b, respectively, of the switch circuit 60 and starter terminals 74and 76. The starter terminals 74 and 76 permit exterior control over theoperation of the starter 22. For most starters 22, when the terminals 74and 76 are short circuited (electrically connected together) the starter22 energizes and operates the associated electrical device 24.Alternatively, when the terminals 74 and 76 are open circuited (isolatedfrom each other), the starter 22 de-energizes, or otherwise ceases theoperation of the associated electrical device 24. Accordingly, the openor short circuited circuit conditions applied between the outputterminals 67a and 67b of the switch circuit 60 connected to the wires 70and 72 are suitable to control the starter 22. The switch circuit 60 mayalternatively be constructed to provide whatever signal or circuitcondition is necessary to control the particular starter 22, which mayinclude a voltage signal, a current signal, digital signal, etc. A lightemitting diode 64 is electrically connected to the switch circuit 60 andis illuminated when the transmission lines 61 and 63 are powered.

Referring to FIG. 3, an electrical schematic diagram of a current sensor300 suitable to provide a full scale 0 volt to 5 volt output signal isshown. The transformer 34 encircles a power cable 23c, producing avoltage between the ends 40a and 40b of its secondary winding. The ends40a and 40b of the transformer secondary winding are connected to theinput circuit 50 which includes a full wave rectifier 312, connected toa variable resistance 314 and associated capacitors 316 and 318, toscale the output of the full wave rectifier 312 to the desired range.The preferred range to interface with conventional control panels 20 is0 volts when no current within the power cable 23c is sensed to 5 voltswhen the maximum desired level within the power cable is sensed.

Referring to FIG. 4, current sensor 320 provides either an open circuitor short circuit at its output terminals 41a and 41b depending onwhether the voltage signal produced in response to the current sensed bythe transformer 34 surpasses a predetermined threshold level. A variableresistor 326 sets the threshold level.

Referring to FIG. 5, current sensor 340 provides a 4-20 ma variableoutput signal at its output terminals 41a and 41b. When the currentsensed in the power cable 23c is 0 then the sensor circuit 340 puts outa 4 ma signal. When the current sensed in the power cable 23c is equalto a desired maximum, when the variable resistor 341 is correctly set,then the sensor circuit 340 puts out a 20 ma signal.

Referring to FIG. 6, an electrical schematic of a switch circuit 60 isshown. A voltage or current signal from the control panel 20 is providedto the input terminals 66 and 68 of the switch circuit 60. When anon-zero signal is received by the switch circuit 60 alight-emitting-diode 64 is illuminated to indicate that the switchcircuit 60 is energized. A diode 368, a resistor 370 and a capacitor 372rectify the signal received at input terminals 66 and 68 if it is analternating signal. The voltage imposed across the capacitor 372 is theinput to the direct current relay 374. If the signal received at inputterminals 66 and 68 is a direct voltage or current signal, then thesignal will also pass through to the relay 374. Accordingly, the switchcircuit 360 is suitable to receive both an alternating signal or directsignal. The relay 374 is energized by a high voltage signal at the inputterminals 66 and 68 and thereby its output contacts 67a and 67b areshorted. When the high voltage signal provided to the relay 374 is belowa threshold level, the output contacts 67a and 67b to the relay 374open, open-circuiting the output contacts 67a and 67b. The outputcontacts 67a and 67b are connected to the terminals 74 and 76 of aremotely located starter 22 (not shown) via wires 70 and 72 (FIG. 1). Inother words, the starter 22 is spaced apart from the switch circuit 60.

Referring to FIG. 7, an alternative switch circuit 60a includes a pairof input terminals 66 and 68, a resistor 406, a diode 407, and acapacitor 408 to permit the use of either an alternating signal or adirect signal as the input to the input terminals 66 and 68. Anopto-isolator 410 isolates the high voltage to the input terminals 66and 68 from the output terminals 67a and 67b for safety. A triac 412,which is a switching device, is energized with a low voltage on the gate413 of the triac 412 to close the triac 412 creating a short circuitbetween the output terminals 67a and 67b. A "snubber circuit" includes aresistor 418 and capacitor 420 connected in parallel across the outputterminals 67a and 67b. In general, the `snubber circuit` prevents falsetriggering of the triac 412 that may occur when driving an inductiveload. Output terminals 67a and 67b are thus short circuited or opencircuited (by the operation of the triac 412) with the result that theterminals 67a and 67b exhibit a circuit condition to the starter 22indicative of whether the electrical device 24 controlled by the starter22 should be operating.

The terms and expressions which have been employed in the foregoingspecification are used therein as terms of description and not oflimitation, and there is no intention, in the use of such terms andexpressions, of excluding equivalents of the features shown anddescribed or portions thereof, it being recognized that the scope of theinvention is defined and limited only by the claims which follow.

What is claimed is:
 1. A protection device for monitoring current in apower cable to an electrical device and for controlling a starter forthe electrical device in response to a system remotely locatedcontroller, the protection device comprising:(a) a transformermagnetically linked with said power cable connected to said electricaldevice, said transformer producing a voltage signal in response to thepresence of a changing current within said power cable; (b) an inputcircuit located proximate to said transformer and having an outputterminal and being electrically connected to said transformer so as toreceive said voltage signal, said input circuit producing, in responseto receiving said voltage signal, one of a first signal representativeof said changing current, and a first circuit condition at said outputterminal of said input circuit representative of said changing currentin said power cable; (c) a switch circuit for sensing one of a secondsignal and a second circuit condition of a remotely located systemcontroller and providing one of a third signal and a third circuitcondition, in response to sensing one of said second signal and saidsecond circuit condition, said third signal and said third circuitcondition being effective to control said starter when said starter iselectrically connected thereto; and, (d) all of said transformer, saidinput circuit, and said switch circuit being located in a single unitarypackage.
 2. The protection device of claim 1 wherein said transformerincludes a wire-wrapped toroidal core at least partially encircling saidpower cable.
 3. The protection device of claim 1 where said toroidalcore has a low magnetic permeability.
 4. The protection device of claim1 wherein said first circuit condition is one of a short circuit and anopen circuit.
 5. The protection device of claim 1 wherein said systemcontroller is a programmable logic device.
 6. The protection device ofclaim 1 wherein said second signal is a direct current signal.
 7. Theprotection device of claim 1 wherein said second signal is a alternatingcurrent signal.
 8. The protection device of claim 1 wherein said secondsignal is a direct voltage signal.
 9. The protection device of claim 1wherein said second signal is an alternating voltage signal.
 10. Theprotection device of claim 1 wherein said switch circuit includes arelay electrically connected to said system controller to sense said oneof a second signal and second circuit condition.
 11. The protectiondevice of claim 1 wherein said switch circuit includes a triacelectrically connected to said system controller to sense said one of asecond signal and second circuit condition.
 12. The protection device ofclaim 1 wherein said switch circuit includes an opto-isolator device tooptically isolate said system controller from said starter.
 13. Theprotection device of claim 1 wherein said switch circuit includes arectifier circuit to rectify said second signal.
 14. The protectiondevice of claim 1 wherein said third circuit condition is a shortcircuit.
 15. The protection device of claim 1 wherein said third circuitcondition is an open circuit.
 16. The protection device of claim 1wherein said first signal is a current signal.
 17. The protection deviceof claim 16 wherein said current signal has a range of magnitude fromabout 4 ma to about 20 ma.
 18. The protection device of claim 1 whereinsaid first signal is a voltage signal.
 19. The protection device ofclaim 18 wherein said voltage signal has a range of magnitude from about0 volts to about 5 volts.